The present invention relates generally to the field of disc drive systems, and more particularly to identifying defective servo information elements in disc drive systems.
Modern disc drives typically comprise a stack of magnetic discs that are coated with a magnetizable medium and mounted on a hub of a spindle motor for rotation at a constant high speed. Data is stored on the discs in a plurality of concentric circular tracks extending approximately from the inner diameter (xe2x80x9cIDxe2x80x9d) of each disc to the outer diameter (xe2x80x9cODxe2x80x9d) of each disc. An array of transducers (xe2x80x9cheadsxe2x80x9d) is typically mounted to flexures at the end of radial actuator arms that extend from an actuator body.
Typically, the heads in a disc drive write data to a selected data track on the disc surface by selectively magnetizing portions of the data track through the application of a time varying write current to the head. In order to subsequently read back the data stored on the data track, the head detects flux transitions in the magnetic fields of the data track and converts these to a signal that is decoded by read channel circuitry of the disc drive.
Control of the position of the heads is typically achieved with a closed loop servo system, such as disclosed in U.S. Pat. No. 5,262,907 entitled xe2x80x9cHard Disc Drive With Improved Servo Systemxe2x80x9d issued to Duffy et al., assigned to the assignee of the present invention. In such a system, head position (servo) information is prerecorded on at least one surface of each disc. The servo system can be a xe2x80x9cdedicated servo systemxe2x80x9d in which one entire disc surface is prerecorded with servo information elements and a corresponding dedicated servo head is used to provide essentially continuous servo position information to the servo system. Alternatively, an xe2x80x9cembeddedxe2x80x9d servo system can be implemented in which servo information elements (e.g., servo wedges) are interleaved with user data and read by the same heads used to read and write the user data.
With either a dedicated or embedded servo system, it is common to generate a position error signal (PES), which is indicative of the position of the head with respect to the center of a particular track. Particularly, during track following, the servo system generates the PES from the servo information read from the disc and uses the PES to generate a correction signal. The correction signal is provided to a power amplifier to control the amount of current through the actuator coil in order to adjust the position of the head.
Typically, the PES is presented as a position dependent signal having a magnitude indicative of the relative distance between the head and the center of the track and a polarity indicative of the direction of the head with respect to the track center. Thus, it is common for the PES to have values ranging from, for example, minus 0.5 to plus 0.5 as the head sweeps across the track and to have a value of zero when the head is centered on the track. It will be recognized that the servo system generates the PES by comparing the relative signal strengths of precisely located magnetized servo burst fields in the servo information on the disc surface. The servo burst fields are generally arranged in an xe2x80x9coffset checkerboardxe2x80x9d pattern relative to a disc track. Through evaluation of the read signal magnitudes received by the servo system as the fields are read, the servo system determines and subsequently controls the relative position of the head to a particular track center. In digital servo systems, the PES is generated as a sequence of digital values over a selected range, with the digital value of any particular sample time indicative of the relative position of a head with respect to a selected track.
The continuing trend in the disc drive industry is to develop products with ever increasing arcal densities and decreasing access time. As this trend continues, greater demands are being placed on the ability of a modem servo system to control the position of data heads with respect to tracks. However, defective servo data in embedded servo hard disc systems, for example, can provide erroneous servo information to the servo system, increasing the possibility of read or write failure. For example, defective PES data could cause a read head to mis-track (e.g., the erroneous servo information can cause the read head to move off-track enough to overlap another track), thereby causing read errors. Even if the defective PES data in the first track does not individually exceed error tolerances, its contribution to the closed-loop system can cause correct PES data in another servo wedge to exceed the error tolerances.
Existing methods for identifying defective servo information elements include rewriting all servo information on a disc or track when a defect is found, and masking any track or servo servo information element that appears to exhibit a defect (i.e., fails to satisfy a predetermined criterion). However, the xe2x80x9crewritexe2x80x9d methods is time-consuming, and therefore, expensive, and the simple xe2x80x9cmaskingxe2x80x9d methods fail to consider the closed-loop characteristic of servo data, and therefore can mask a servo wedge that is merely a symptom of another (probably preceding) defective wedge. Accordingly, defective servo data in a first servo wedge may not fail to satisfy an error criterion for an individual servo wedge, but it may contribute to an apparent defect in a subsequent servo wedge in the track. Other existing xe2x80x9cmaskingxe2x80x9d methods attempt to recover a track appearing to have a PES defect by masking an alternate servo wedge in a track in the vicinity of the apparent PES defect wedge. Disadvantages of such methods include a failure to provide customized recovery steps in accordance with a specific detected error type, and the absence of partial recovery steps for defective servo information elements in accordance with error type.
The present invention provides a method and disc drive system for identifying defective servo information elements on a disc.
In accordance with a preferred embodiment, a method for identifying a defective servo information element in a track on a disc is provided including reading servo information elements from the track on the disc to receive first servo data status values associated with the servo information elements. If the first servo data status value associated with a first servo information element fails to satisfy a first predetermined criterion, the first servo information element is identified as a suspected servo information element. Location and error type information associated with the suspected servo information element is recorded. The servo information elements from the track on the disc are reread with a second servo information element masked, and second servo data status values for the servo information elements are received. If the second servo data status value associated with the suspected servo information element satisfies the first predetermined criterion, the second servo information element is identified as the defective servo information element.
In accordance with a preferred embodiment a disc drive system for identifying a defective servo information element In a track on a disc is provided including a read head that reads servo information elements from the track on the disc. If a first servo data status value associated with the first servo information element falls to satisfy a predetermined criterion, a test module identifies a first servo information element as a suspected servo information element. A storage module stores location and error type information associated with the suspected servo information element. A recovery module masks a second servo information element and rereads the servo information elements to receive second servo data status values for the servo information elements. If the second servo data status value associated with the suspected servo information element satisfies the predetermined criterion, the recovery module identifies the second servo information element as the defective servo information element.
In accordance with another embodiment, a disc drive system for identifying a defective servo information element in a track on a disc is provided including a read head that reads servo formation elements associated with servo data status values from the disc; and means for identifying the defective servo information element in accordance with the servo data status values.
These and various other features as well as advantages that characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.